Abstract:

A method for producing an optical element, in which a multi-layered film
is provided on a plastic substrate, having a resistance against lights in
a wavelength range of 300 nm to 450 nm, is provided. The method for
producing the optical element according to the invention, is an method
for producing an optical element having a multi-layered film in which a
layer made of a low-refractive-index material and a layer made of a
high-refractive-index material are alternately formed on a plastic
substrate, the optical element being used for light in a wavelength range
of 300 nm to 450 nm. The optical element producing method includes the
steps of forming alternately the layer made of the low-refractive-index
material and the layer made of the high-refractive-index material on the
plastic substrate to produce the optical element while plasma or ionized
gas is generated by a generation source in forming at least the layer
made of the high-refractive-index material under a predetermined
producing conditions; measuring an oxygen permeability coefficient of the
produced optical element; and changing one of an output of the generation
source, an ambient gas pressure in forming the layer made of the
low-refractive-index material, and an ambient gas pressure in forming the
layer made of the high-refractive-index material among the predetermined
producing conditions when the oxygen permeability coefficient of the
produced optical element is more than 30 cm3mm/(m224 hratm).

Claims:

1. An method for producing an optical element having a multi-layered film
in which a layer made of a low-refractive-index material and a layer made
of a high-refractive-index material are alternately formed on a plastic
substrate, the optical element being used for light in a wavelength range
of 300 nm to 450 nm,the optical element producing method comprising the
steps of:forming alternately the layer made of the low-refractive-index
material and the layer made of the high-refractive-index material on the
plastic substrate to produce the optical element while plasma or ionized
gas is generated by a generation source in forming at least the layer
made of the high-refractive-index material under predetermined producing
conditions;measuring an oxygen permeability coefficient of the produced
optical element; andchanging one of an output of the generation source,
an ambient gas pressure in forming the layer made of the
low-refractive-index material, and an ambient gas pressure in forming the
layer made of the high-refractive-index material among the predetermined
producing conditions when the oxygen permeability coefficient of the
produced optical element is more than 30 cm3mm/(m224 hratm).

2. The optical element producing method according to claim 1, wherein an
ion plating method is adopted, andthe optical element producing method
includes the steps of:forming alternately the layer made of the
low-refractive-index material and the layer made of the
high-refractive-index material on the plastic substrate to produce the
optical element while the plasma is generated by a high-frequency power
supply in forming at least the layer made of the high-refractive-index
material under the predetermined producing conditions;measuring the
oxygen permeability coefficient of the produced optical element;
andchanging one of an output of the high-frequency power supply, the
ambient gas pressure in forming the layer made of the
low-refractive-index material, and the ambient gas pressure in forming
the layer made of the high-refractive-index material among the
predetermined producing conditions when the oxygen permeability
coefficient of the produced optical element is more than 30
cm3mm/(m224 hratm).

3. The optical element producing method according to claim 2, wherein
oxygen is not used as ambient gas in forming the layer made of the
low-refractive-index material.

4. The optical element producing method according to claim 3, wherein
inert gas is used as the ambient gas in forming the layer made of the
low-refractive-index material.

5. The optical element producing method according to claim 2, wherein
oxygen is not used as ambient gas in forming the layer made of the
high-refractive-index material.

6. The optical element producing method according to claim 5, wherein
inert gas is used as the ambient gas in forming the layer made of the
high-refractive-index material.

7. The optical element producing method according to claim 2, wherein the
plasma is generated in forming the layer made of the
high-refractive-index material.

8. The optical element producing method according to claim 1, wherein a
sputtering method is adopted, andthe optical element producing method
includes the steps of:forming alternately the layer made of the
low-refractive-index material and the layer made of the
high-refractive-index material on the plastic substrate to produce the
optical element while oxygen is supplied to generate oxygen plasma by an
output of an oxidation source in a oxidation region oxygen;measuring the
oxygen permeability coefficient of the produced optical element;
andchanging one of an output of the oxidation source when the oxygen
permeability coefficient of the produced optical element is more than 30
cm3mm/(m224 hratm).

9. The optical element producing method according to claim 1, wherein an
ion beam assist deposition method is adopted, andthe optical element
producing method includes the steps offorming alternately the layer made
of the low-refractive-index material and the layer made of the
high-refractive-index material to produce the optical element while an
ion gun ionizes ambient gas to supply ionized gas onto the plastic
substrate under the predetermined producing conditions;measuring the
oxygen permeability coefficient of the produced optical element;
andchanging one of an output of the ion gun, the ambient gas pressure in
forming the layer made of the low-refractive-index material, and the
ambient gas pressure in forming the layer made of the
high-refractive-index material among the predetermined producing
conditions when the oxygen permeability coefficient of the produced
optical element is more than 30 cm3mm/(m224 hratm).

Description:

TECHNICAL FIELD

[0001]The present invention relates to a method for producing an optical
element, in which a multi-layered film is provided on a substrate, having
a resistance against lights in a wavelength range of 300 nm to 450 nm. In
particular, the present invention relates to a method for producing an
optical element with a multi-layered film which can be used in a
short-wavelength, high-power (30 mW/mm2 or more) blue laser.

BACKGROUND ART

[0002]A short-wavelength, high-power blue laser is expected to be more
widely used in an optical pickup or the like. In general, plastic is
easily damaged by a laser. Therefore, some optical parts of devices using
a short-wavelength, high-power blue laser consist of glass instead of
plastic to avoid damage by a laser. For this reason, the prices of
devices are relatively high. The relatively high prices are a hurdle to
be overcome to enlarge the market of the devices.

[0003]At present, plastic materials which can cope with a
relatively-low-power blue laser are supplied from various material
manufacturers. However, there is no plastic material which can withstand
a high-power blue laser.

[0004]On the other hand, an antireflection film is frequently formed on
surfaces of plastic lenses used in video camcorders, still cameras,
glasses, and the like. Such an antireflection film consists of a
multi-layered film obtained by alternatively forming low-refractive-index
layers and high-refractive-index layers. The multi-layered film is
described in Japanese Patent Applications Laid-Open Nos. 11-30703,
11-171596, 11-326634, 2002-71903, 2003-98308, 2003-248102, and the like.
However, a conventional antireflection film cannot prevent a damaged
caused by a high-power blue laser or ultraviolet light.

[0005]Generally an optical element in which a multi-layered film is
provided on a plastic substrate cannot be used for light having a
wavelength shorten than 300 nm because a transmittance of the light for
the plastic substrate is remarkably lowered. On the other hand, the
optical element can be used without any difficulty for lights having
wavelengths longer than 450 nm because of little deterioration (damage)
of the plastic substrate caused by the light. Therefore, there is
generated deterioration (damage) of the plastic substrate of the optical
element in which the multi-layered film is provided on the plastic
substrate becomes troublesome for lights in a wavelength range of 300 nm
to 450 nm.

[0006]Under the background art mentioned above, there is a need of a
method for producing an optical element, in which a multi-layered film is
provided on a plastic substrate, having a resistance against lights in a
wavelength range of 300 nm to 450 nm.

DISCLOSURE OF THE INVENTION

[0007]A method for producing the optical element according to the
invention, is a method for producing an optical element having a
multi-layered film in which a layer made of a low-refractive-index
material and a layer made of a high-refractive-index material are
alternately formed on a plastic substrate, the optical element being used
for light in a wavelength range of 300 nm to 450 nm. The optical element
producing method includes the steps of forming alternately the layer made
of the low-refractive-index material and the layer made of the
high-refractive-index material on the plastic substrate to produce the
optical element while plasma or ionized gas is generated by a generation
source in forming at least the layer made of the high-refractive-index
material under a predetermined producing conditions; measuring an oxygen
permeability coefficient of the produced optical element; and changing
one of an output of the generation source, an ambient gas pressure in
forming the layer made of the low-refractive-index material, and an
ambient gas pressure in forming the layer made of the
high-refractive-index material among the predetermined producing
conditions when the oxygen permeability coefficient of the produced
optical element is more than 30 cm3mm/(m2024 hratm).

[0008]In the optical element producing method according to an embodiment
of the invention, an ion plating method is adopted, and the optical
element producing method includes the steps of forming alternately the
layer made of the low-refractive-index material and the layer made of the
high-refractive-index material on the plastic substrate to produce the
optical element while the plasma is generated by a high-frequency power
supply in forming at least the layer made of the high-refractive-index
material under the predetermined producing conditions; measuring the
oxygen permeability coefficient of the produced optical element; and
changing one of an output of the high-frequency power supply, the ambient
gas pressure in forming the layer made of the low-refractive-index
material, and the ambient gas pressure in forming the layer made of the
high-refractive-index material among the predetermined producing
conditions when the oxygen permeability coefficient of the produced
optical element is more than 30 cm3mm/(m2024 hratm).

[0009]In the optical element producing method according to an embodiment
of the invention, a sputtering method is adopted, and the optical element
producing method includes the steps of forming alternately the layer made
of the low-refractive-index material and the layer made of the
high-refractive-index material on the plastic substrate to produce the
optical element while oxygen is supplied to generate oxygen plasma by an
output of an oxidation source in a oxidation region; measuring the oxygen
permeability coefficient of the produced optical element; and changing an
output of the oxidation source when the oxygen permeability coefficient
of the produced optical element is more than 30 cm3mm/(m2024
hratm).

[0010]In the optical element producing method according to an embodiment
of the invention, an ion beam assist deposition method is adopted, and
the optical element producing method includes the steps of forming
alternately the layer made of the low-refractive-index material and the
layer made of the high-refractive-index material to produce the optical
element while an ion gun ionizes ambient gas to supply ionized gas onto
the plastic substrate under the predetermined producing conditions;
measuring the oxygen permeability coefficient of the produced optical
element; and changing one of an output of the ion gun, the ambient gas
pressure in forming the layer made of the low-refractive-index material,
and the ambient gas pressure in forming the layer made of the
high-refractive-index material among the predetermined producing
conditions when the oxygen permeability coefficient of the produced
optical element is more than 30 cm3mm/(m224 hratm).

BRIEF DESCRIPTION OF DRAWINGS

[0011]FIG. 1 is a diagram showing a configuration of an optical element
having a laser damage suppression film according to a first embodiment of
the present invention;

[0012]FIG. 2 is a graph showing a result obtained by measuring a rate of
change of light intensity of an optical element after a blue laser is
irradiated on the optical element for 1000 hours;

[0013]FIG. 3 is a graph showing results obtained by measuring total
wavefront aberrations (RMS) of the optical element before and after the
blue laser is irradiated on the optical element for 1000 hours;

[0014]FIG. 4 is a diagram showing a configuration of an ion plating
apparatus to practice an ion plating method;

[0015]FIG. 5 is a graph showing an amount of change in light transmittance
of an optical element obtained by forming a conventional film on a
conventional substrate and an optical element obtained by forming
improved films 1 and 2 on the conventional substrate;

[0016]FIG. 6 is a graph showing oxygen permeability coefficients of the
optical element obtained by forming the conventional film on the
conventional substrate and the optical element obtained by forming
improved films 1 and 2 on the conventional substrate;

[0017]FIG. 7 is a graph showing a change in chemiluminescence quantity
after a blue laser is irradiated on the optical element obtained by
forming the conventional film on the conventional substrate and the
optical element obtained by forming improved film 1 on the conventional
substrate and then stopped;

[0018]FIG. 8 is a graph showing a part of FIG. 7 in which a time axis is
enlarged;

[0019]FIG. 9 is a graph showing fluorescence quantities of the
conventional substrate and the substrate molded in a nitrogen atmosphere
at a wavelength of about 320 nm in excitation at a wavelength of 280 nm;

[0020]FIG. 10 is a graph showing amounts of change in light transmittance
of the conventional substrate and a substrate molded in a nitrogen
atmosphere;

[0021]FIG. 11 is a graph showing amounts of change in light transmittance
of the optical element obtained by forming improved film 1 on the
conventional substrate and the optical element obtained by forming
improved film 1 on the substrate molded in a nitrogen atmosphere;

[0022]FIG. 12 is a flowchart illustrating a method for changing producing
conditions to determine conditions for producing an optical element
having a small oxygen permeability coefficient;

[0023]FIG. 13 illustrates a change in oxygen permeability coefficient of
the optical element when an output of a high-frequency power supply is
changed;

[0024]FIG. 14 illustrates a change in oxygen permeability coefficient of
the optical element when an oxygen pressure value is changed in forming a
high-refractive-index material;

[0025]FIG. 15 illustrates a change in oxygen permeability coefficient of
the optical element when an argon pressure value is changed in forming a
low-refractive-index material;

[0026]FIG. 16 illustrates a configuration of a multi-layered film having
an L-layer structure;

[0027]FIG. 17 illustrates an oxygen permeability coefficient of an optical
element in which improved films 3 to 5 are formed on a conventional
substrate;

[0028]FIG. 18 illustrates a change in light transmittance of an optical
element in which an improved film 2 is formed on the conventional
substrate;

[0029]FIG. 19 illustrates a change in light transmittance of an optical
element in which an improved film 4 is formed on the conventional
substrate;

[0030]FIG. 20 illustrates a change in light transmittance of an optical
element in which an improved film 5 is formed on the conventional
substrate;

[0031]FIG. 21 illustrates a configuration of a sputtering apparatus in
film forming is performed by sputtering;

[0032]FIG. 22 is a view for explaining a principle of the film forming
performed by the sputtering;

[0033]FIG. 23 illustrates a change in oxygen permeability coefficient of
an optical element when an output of ECR of the sputtering apparatus is
changed in forming films;

[0034]FIG. 24 illustrates a relationship between an oxygen permeability
coefficient and an amount of change in light transmittance; and

[0035]FIG. 25 illustrates a configuration of an ion beam assist deposition
apparatus in which film forming is performed by an ion beam assist
deposition method.

DETAILED DESCRIPTION OF THE INVENTION

[0036]FIG. 1 is a diagram showing a configuration of an optical element
having a laser damage suppression film according to a first embodiment of
the present invention. In FIG. 1, a layer 103 consisting of silicon
monoxide (SiO) is formed on a substrate 101 consisting of a
blue-laser-coping plastic material. The layer 103 consisting of silicon
monoxide functions to improve adhesiveness between the substrate 101
consisting of a plastic material and a layer formed thereon. On the layer
103 consisting of silicon monoxide, layers 105 consisting of a
low-refractive-index material and layers 107 consisting of a
high-refractive-index material are alternately formed. In the embodiment,
three layers 105 consisting of low-refractive-index material and two
layers 107 consisting of high-refractive-index material are formed.

[0037]In this case, the blue-laser-coping plastic material is an
olefin-based material. More specifically, the material is a thermoplastic
transparent resin cycloolefin polymer having an antioxidant function.

[0038]The layer 103 consisting of silicon monoxide is formed on the
substrate 101 by a vacuum deposition method. In the vacuum deposition
method, a material (in this case, silicon monoxide) to be formed as a
thin film, is heated by a resistance wire or by electron beam irradiation
to evaporate the material. The evaporated material is caused to attach to
(deposited on) the substrate to form a thin film. The thickness of the
layer 103 consisting of silicon monoxide is about several hundred
nanometers.

[0039]The low-refractive-index material is silicon dioxide (SiO2) in
this embodiment. The refractive index of the layer 105 consisting of
silicon dioxide is 1.4 to 1.5. The layer 105 consisting of silicon
dioxide is formed by the vacuum deposition method. The thickness of the
layer 105 consisting of silicon dioxide is from several ten nanometers to
several hundred nanometers.

[0040]Aluminum fluoride (AlF3), magnesium fluoride (MgF2), and a
composite oxide including silicon dioxide (SiO2) can also be used as
the low-refractive-index material.

[0041]The high-refractive-index material is obtained by adding a slight
amount of titanium dioxide (TiO2) to tantalum pentoxide
(Ta2O5) in the embodiment. The refractive index of the layer
107 consisting mainly of tantalum pentoxide is 2.0 to 2.3. The layer 107
consisting mainly of tantalum pentoxide is formed by an ion plating
method. The ion plating method is a method for ionizing some evaporated
particles by using gas plasma and depositing them on a substrate biased
to a negative high voltage. Since the material to be deposited is
accelerated by an electric field and attached to the substrate, a film
having high adherence is obtained. A thickness of the layer 107
consisting mainly of tantalum pentoxide is several ten nanometers to
several hundred nanometers.

[0042]As the material of the layer 107, TaxOy where values of x
and y are properly determined can be used.

[0043]As the high-refractive-index material, a titanium-oxide-based
material can also be used.

[0044]Layers having different refractive indexes may be alternately formed
to constitute a large number of reflective surfaces. External lights
reflected by the large number of reflective surfaces may interfere with
one another and may be canceled out to obtain an antireflection effect.
Optical path lengths (=layer thicknesses and refractive indexes) of the
layers may be different from one another to generate interference in a
wide wavelength range, so that an antireflection effect can be obtained
in a wide wavelength range of external lights. In this manner, the
multi-layered film may include not only the laser damage suppression
function but also the antireflection function.

[0045]FIG. 4 is a diagram showing a configuration of an ion plating
apparatus to practice the ion plating method. The ion plating apparatus
is disclosed in Japanese Patent Application Laid-Open No. 1-48347, for
example. In a vacuum chamber 412, a base holder 407 consisting of a
conductive material and supporting a base 408, and a support member
consisting of a conductive material and supporting the base holder via an
insulating material, constitute a capacitor 406.

[0046]A high-frequency power supply 401 is connected between the vacuum
chamber 412 and the base holder 407 via a blocking capacitor 403 and a
matching box 402 to supply a high-frequency voltage. A DC power supply
404 is connected between the vacuum chamber 412 and the base holder 407
via a choke coil 405 such that the base holder 407 is on a negative side
to supply a DC bias voltage. An output from the high-frequency power
supply 401 is 500 W, and a voltage of the DC power supply 404 is 100 V.

[0047]An output from the high-frequency power supply 401 preferably ranges
from 300 to 900 W. In this range, an output value can be adjusted to
improve the denseness of the film.

[0048]The capacitor 406 operates together with the matching box 402
connected to the high-frequency power supply 401 which supplies a
high-frequency voltage into the vacuum chamber 412 to perform matching,
and thus a stable electric field can be made and maintained between a
material to be evaporated 409 on a resistor heating board 410 and the
base 408. As a result, a thin film having high purity, high density, and
high adhesiveness can be formed on the surface of the base 408.

[0049]An electron gun 4101 for electron beam heating is installed under a
crucible including the resistor heating board 410.

[0050]An outline of a film forming method will be shown in the following
table.

[0054]In film formation, oxygen is introduced into the vacuum chamber 412
by a valve (not shown). The oxygen introduction pressure setting is
setting of an oxygen pressure in the chamber. An oxygen partial pressure
preferably ranges from 3.0×10-3 to 5.0×10-2 Pa.
When the oxygen partial pressure is adjusted in this range, a rate of
change of light intensity of an optical element (to be described later)
can be set at an appropriate value. A gas in the vacuum chamber 412 is
exhausted from an exhaust port 411.

[0055]A function of the plasma will be described below. The plasma is
generated in the vacuum chamber 412 by the high-frequency power supply
401. When particles of the evaporated material pass through the plasma,
the particles become in the ionized state. The ionized particles are
assisted by the plasma, and the particles impinge energetically on the
base 408 to be deposited on the base 408. Therefore, the dense film is
formed.

[0056]As described later in detail, an ambient gas pressure has an
influence on the plasma generation, and there is a gas pressure at which
gas is ionized to efficiently produce the plasma.

[0057]Three kinds of optical elements (comparative examples 1 to 3)
illustrated in following Table 2 are prepared in order to compare with
the optical element of the embodiment of FIG. 1. The comparative example
3 is the optical element made of blue-laser-compatible plastic in which
coating is not performed to the surface.

[0058]FIG. 2 is a graph showing a result obtained by measuring a rate of
change of light intensity of an optical element after a blue laser is
irradiated on the optical element at an ambient temperature of 25°
C. for 1000 hours. The energy density of blue laser irradiation is about
120 mW/mm2. In this case, a rate of change of light intensity of the
optical element can be expressed by the following equation.

Rate of change of light intensity=((transmittance after
irradiation/transmittance before irradiation)-1)100%

As an example, when the transmittance before irradiation is 90% and the
transmittance after irradiation is 80%, the rate of change of light
intensity is given by:

((0.80/0.90)-1)100=-11.1%

[0059]Reference symbol B in FIG. 2 indicates a measurement result of a
rate of change of light intensity of the optical element according to the
embodiment. Reference symbol A in FIG. 2 indicates a measurement result
of a rate of change of light intensity of the first comparative example.
Reference symbol C in FIG. 2 indicates a measurement result of a rate of
change of light intensity of the second comparative example. Reference
symbol D in FIG. 2 indicates a measurement result of a rate of change of
light intensity of an optical element according to the third comparative
example. The optical element according to the third comparative example
is irradiated with a blue laser in a nitrogen atmosphere.

[0060]FIG. 3 is a graph showing results obtained by measuring total
wavefront aberrations (RMS) of optical elements before and after the blue
laser is irradiated on the optical element at an ambient temperature of
25° C. for 1000 hours. An energy density of blue laser irradiation
is about 120 mW/mm2.

[0061]The total wavefront aberration is obtained by expressing a
misalignment of a wavefront from a reference sphere by a standard
deviation. In this case, the reference sphere is a sphere which
intersects with an optical axis at the center of entrance and exit pupils
such that a principal ray is focused on. In measurement of the total
wavefront aberration, an interference band is generated by an
interferometer. The wavefront aberration is calculated from a map of the
interference band.

[0062]Reference symbols B1 and B2 in FIG. 3 indicate measurement results
of total wavefront aberrations of the optical element according to the
present embodiment. Reference symbols A1 and A2 in FIG. 3 indicate
measurement results of total wavefront aberrations in the first
comparative example. Reference symbols C1 and C2 in FIG. 3 indicate
measurement results of total wavefront aberrations in the second
comparative example. Reference symbols D1 and D2 in FIG. 3 indicate
measurement results of total wavefront aberrations of the optical element
according to the third comparative example. The optical element according
to the third comparative example is irradiated with a blue laser in a
nitrogen atmosphere. Reference symbols A1, B1, C1, and D1 indicate
measurement results of total wavefront aberrations before the blue laser
is irradiated, while reference symbols A2, B2, C2, and D2 indicate
measurement results of total wavefront aberrations after the blue laser
is irradiated.

[0063]The following points will be apparent from the measurement results
in FIGS. 2 and 3. In the present embodiment, even after it has been
irradiated with a high-power blue laser for 1000 hours, the light
intensity remains almost unchanged. Furthermore, the total wavefront
aberrations after the irradiation remain almost unchanged in comparison
with those before the irradiation.

[0064]The rate of change of light intensity decreases by about 10% in the
first comparative example (A in FIG. 2), about 20% in the second
comparative example (C in FIG. 2), and about 5% in the third comparative
example (D in FIG. 2). In the second comparative example in which the ion
plating method is not used in formation of a high-refractive-index layer
(C in FIG. 2), the rate of change of light intensity is high. More
specifically, a light transmittance intensity of the optical element
considerably decreases. The light transmittance intensity of the optical
element decreases for the following reason. That is, when a high-power
blue laser is irradiated for a long period of time, chemical bonds of the
plastic serving as a polymer may be broken (damaged) to change a bonding
state. When the ion plating method is used in formation of a
high-refractive-index layer, the above damage is suppressed.

[0065]When the coat-less optical element according to the third
comparative example is placed in a nitrogen atmosphere, a decrease in
light transmittance intensity is relatively small. Then, it is inferred
that a material except for nitrogen in the air should accelerate damage
of the optical element. Therefore, it can be considered that a rate of
mixing the material in the air which accelerates damage of the optical
element into the optical element can be decreased by using the ion
plating method in formation of a high-refractive-index layer.

[0066]The total wavefront aberration obtained after irradiation of the
high-power blue laser is about 2.5 times that in the first comparative
example (A1 and A2 in FIG. 3) in which a PMMA-based plastic is used. The
total wavefront aberration obtained after irradiation of a high-power
blue laser remains almost unchanged in the second and third comparative
examples. For this reason, in the optical element using a
blue-laser-coping plastic, it is considered the shape of the optical
element surface remains almost unchanged. On the other hand, in the
optical element using the PMMA-based plastic, it is considered the total
wavefront aberration may increase because the shape of the optical
element surface has changed.

[0067]In the embodiment, films are formed by the ion plating method.
However, the film may be formed while a plasma state is generated by a
plasma CVD method, an ion beam assist deposition method, sputtering
method or the like.

[0068]The present invention is characterized in that a film is formed on a
substrate consisting of a blue-laser-coping plastic material by a plasma
generating method such as an ion plating method.

[0069]According to the characteristic, the above remarkable effect is
achieved with respect to suppression of damage of the optical element
caused by a laser. A mechanism which achieves the effect is considered as
follows.

[0070]A catalytic action which operates to create an action having
oxidation decomposition from moisture or oxygen may be suppressed by
using a substrate consisting of a thermoplastic transparent resin
cycloolefin polymer having an antioxidant function and by increasing a
film denseness by film formation by the ion plating method (forming an
oxygen impermeability film). Therefore, it can be presumed that substrate
damage by a blue laser beam is suppressed. A ground for causing this
presumption is also expected from the measurement result (D in FIG. 2) of
a rate of change of light intensity when a laser irradiation experiment
is performed in a nitrogen atmosphere. It is considered that use of
tantalum oxide in film formation by the ion plating method further
improves the denseness of the film.

[0071]As another embodiment, a multi-layered film formed by the following
film forming method will be described below. The multi-layered film
formed by this film forming method is called improved film 1.

[0072]In Table 3, the tantalum oxide based material means a material in
which a small amount of titanium dioxide (TiO2) is added to tantalum
pentoxide (Ta2O5).

[0073]A method for determining a film thickness of each layer included in
the multi-layered film will be described below.

[0074]FIG. 16 illustrates a configuration of a multi-layered film having
an L-layer structure. As shown in FIG. 16, it is assumed n(j) is a
refractive index of a j-th layer and d(j) is a film thickness in a
multi-layered optical thin film system having an L-layer structure. An
optical characteristic can be expressed by the following characteristic
matrix when light having a wavelength λ is incident to the optical
system of FIG. 16 with an incident angle θ. In FIG. 16, I is
incident light.

M=M(L)M(L-1) . . . M(j) . . . M1

[0075]M is a two-by-two matrix, and a matrix of each layer also becomes
the two-by-two matrix. M(j) means a matrix of the j-th layer, and M(j)
can be expressed as follows:

At this point, the wavelength used, the light incident angle, and optical
characteristics specification (reflectivity and transmittance) depend on
required specifications in designing the optical thin film. The
refractive indexes of the substrate, film, and medium are obtained as
measured data by the measurement.

[0082]In designing the multi-layered film, the film thickness of each
layer is determined from the measured data and required specifications
based on the above-described principle so as to satisfy the required
optical characteristics (reflectivity and transmittance).

[0083]A film forming method in Table 3 is different from the film forming
method in Table 1 in that a low-refractive-index layer consisting of
silicon dioxide is formed in an argon atmosphere while generating a
plasma state. An argon partial pressure in formation of the
low-refractive-index layer preferably ranges from 3.0×10-3 to
5.0×10-2 Pa. When the low-refractive-index layer is formed in
an argon atmosphere while generating a plasma state, even though a
substrate is exposed to a high-temperature environment (for example,
85° C.) or a high-temperature and high-humidity environment (for
example, 60° C. and 90%) for a long time, an amount of change in
transmittance is almost zero. The argon atmosphere is more advantageous
than an oxygen plasma atmosphere.

[0084]In the film forming method of Table 3, a high-frequency power supply
for generating the plasma has an output of 500 W. A DC voltage is set at
300V.

[0085]Then, the optical element characteristic is observed by changing the
producing conditions. Specifically the optical element characteristic is
an oxygen permeability coefficient. The reason the oxygen permeability
coefficient is noticed is that, as described later, an amount of change
in light transmittance of the optical element becomes small when the
optical element has the low oxygen permeability coefficient.

[0086]The gas permeability coefficient is generally expressed by the
following equation:

where a unit of oxygen permeability coefficient is cm3mm/(m2024
hratm).

[0087]FIG. 12 is a flowchart illustrating a method for determining
conditions for producing an optical element having a small oxygen
permeability coefficient by changing producing conditions when forming a
multi-layered film on a substrate made of thermoplastic transparent resin
cycloolefin polymer.

[0088]Referring to FIG. 12, in Step S010, initial values of the producing
conditions are determined.

[0089]In Step S020, the optical element is produced according to the
producing conditions.

[0090]In Step S030, the oxygen permeability coefficient of the produced
optical element is measured.

[0091]In Step S040, a determination whether necessary data is obtained is
made. When the necessary data is obtained, the method for determining the
optical element producing conditions is ended. When the necessary data is
not obtained, the flow goes to Step S050.

[0092]In Step S050, the producing conditions are changed.

[0093]FIG. 13 illustrates a change in oxygen permeability coefficient of
the optical element when the output of the high-frequency power supply is
changed. Table 3 illustrates the producing conditions such as the argon
pressure value in forming the low-refractive-index material and the
oxygen pressure value in forming the high-refractive-index material. The
oxygen permeability coefficient of the produced optical element is
largely changed by the output of the high-frequency power supply during
the production, and the oxygen permeability coefficient is minimized in
the case of 500 W to 600 W.

[0094]Oxygen is introduced instead of argon in forming the
low-refractive-index material, and the pressure value is set at
6×10-3 Pa to change the output of the high-frequency power
supply. In such cases, when the change in oxygen permeability coefficient
of the optical element is observed, the oxygen permeability coefficient
is monotonously decreased as the output of the high-frequency power
supply is increased. The oxygen permeability coefficient becomes about
110 cm3mm/(m224 hratm) when the high-frequency power supply has
the output of 0 W, and the oxygen permeability coefficient becomes 40
cm3mm/(m224 hratm) when the high-frequency power supply has the
output of 800 W.

[0095]FIG. 14 illustrates a change in oxygen permeability coefficient of
the optical element when the oxygen pressure value is changed in forming
the high-refractive-index material. Table 3 illustrates the argon
pressure value in forming the low-refractive-index material. The
high-frequency power supply has the output of 500 W. The oxygen
permeability coefficient of the optical element is largely changed by the
oxygen pressure value in forming the high-refractive-index material. The
oxygen permeability coefficient is equal to or lower than 20
cm3mm/(m224 hratm) when the oxygen pressure value is equal to
or lower than 3×10-2 Pa, and the oxygen permeability
coefficient is rapidly increased when the oxygen pressure value is larger
than 3×10-2 Pa.

[0096]The oxygen is introduced instead of the argon in forming the
low-refractive-index material, the pressure value is set at
6×10-3 Pa, and the output of the high-frequency power supply
is set at 500 W to change the oxygen pressure value in forming the
high-refractive-index material. In such cases, when the change in oxygen
permeability coefficient of the optical element is observed, the oxygen
permeability coefficient of the optical element is largely changed by the
oxygen pressure value in forming the high-refractive-index material, and
the oxygen permeability coefficient becomes equal to or lower than 30
cm3mm/(m224 hratm) when the oxygen pressure value is equal to
or lower than 1.5×10-2 Pa in forming the high-refractive-index
material.

[0097]FIG. 15 illustrates a change in oxygen permeability coefficient of
the optical element when the argon pressure value is changed in forming
the low-refractive-index material. Table 3 illustrates the oxygen
pressure value in forming the high-refractive-index material. The
high-frequency power supply has the output of 500 W. The oxygen
permeability coefficient of the optical element is largely changed by the
argon pressure value in forming the low-refractive-index material, and
the oxygen permeability coefficient is minimized in the case of
5×10-3 Pa to 7×10-3 Pa.

[0098]The oxygen pressure value is set at 3×10-2 Pa in forming
the high-refractive-index material, the output of the high-frequency
power supply is set at 500 W, and the oxygen is introduced instead of the
argon in forming the low-refractive-index material to change the oxygen
pressure value. In such cases, when the change in oxygen permeability
coefficient of the optical element is observed, the oxygen permeability
coefficient is monotonously increased as the oxygen pressure value is
increased. The oxygen permeability coefficient is about 60
cm3mm/(m224 hratm) when the oxygen pressure value is
4×10-3 Pa, and the oxygen permeability coefficient is about 80
cm3mm/(m224 hratm) when the oxygen pressure value is
1×10-2 Pa.

[0099]In summary, the optical element having the small oxygen permeability
coefficient is produced under the following conditions. That is, the
high-frequency power supply has the output of 500 W to 600 W, the oxygen
pressure value is equal to or lower than 3×10-2 Pa in forming
the high-refractive-index material, the argon gas is introduced as the
ambient gas in forming the low-refractive-index material, and the argon
pressure value ranges from 5×10-3 Pa to 7×10-3 Pa.
This is attributed to that fact that the optimum plasma, in which the
particles of the film forming material are ionized and deposited, is
generated under the conditions to form the dense film through which the
oxygen is hardly permeated. At this point, in forming the
low-refractive-index material, the inert gas such as the argon gas is
introduced as the ambient gas while the oxygen is not introduced, thereby
advantageously producing the optical element having the small oxygen
permeability coefficient. The conditions of Table 3 are matched with the
above-described conditions.

[0100]The above-described conditions indicate only the numerical range in
the embodiment. Generally, the conditions for producing the optical
element having the small oxygen permeability coefficient can be obtained
by changing the output of the high-frequency power supply, the ambient
gas pressure value in forming the high-refractive-index material, and the
ambient gas pressure value in forming the low-refractive-index material,
according to the method shown in FIG. 12.

[0101]As still another embodiment, a multi-layered film formed by the
following film forming method will be described below. The multi-layered
film formed by this film forming method is called improved film 2.

[0102]In Table 4, the tantalum oxide based material means a material in
which a small amount of titanium dioxide (TiO2) is added to tantalum
pentoxide (Ta2O5).

[0103]Improved film 2 does not include an adhesion layer consisting of
silicon monoxide but includes a layer consisting of a
tantalum-oxide-based material as a first layer on the substrate. The
total film thickness of improved film 1 is 547.5 nm, whereas the total
film thickness of improved film 2 is 182.0 nm. In a diffraction optical
element having a surface on which a fine structure is formed, when a film
thickness is large, influence on the shape of the fine structure
increases. Since improved film 2 is thin, influence on the shape of the
fine structure is small.

[0104]In the film forming method of Table 4, the high-frequency power
supply for generating the plasma has the output of 500 W. The DC voltage
is set at 300V.

[0105]A multi-layered film which has the same structure as that of
improved film 2 and which is formed according to a vacuum deposition
method which does not generate plasma, is called a conventional film.

[0106]Table 5 illustrates film forming conditions for the conventional
film.

[0108]A substrate which is not a substrate molded in a nitrogen atmosphere
(will be described later) and which consists of a thermoplastic
transparent cycloolefin polymer is called a conventional substrate.

[0109]FIG. 5 is a graph showing an amount of change in light transmittance
of an optical element obtained by forming a conventional film on a
conventional substrate and optical elements obtained by forming improved
films 1 and 2 on the conventional substrates. The amounts of change in
light transmittance of the optical elements in which improved films 1 and
2 are formed are considerably improved in comparison with the amount of
change in light transmittance of the optical element in which the
conventional film is formed.

[0110]FIG. 6 is a graph showing oxygen permeability coefficients of the
optical element obtained by forming the conventional film on the
conventional substrate and the optical element obtained by forming
improved films 1 and 2 on the conventional substrate.

[0111]Oxygen permeability coefficients of the optical elements obtained by
forming improved films 1 and 2 on the conventional substrate are smaller
than the oxygen permeability coefficient of the optical element obtained
by forming the conventional film on the conventional substrate. The
optical elements obtained by forming improved films 1 and 2 on the
conventional substrate do not easily transmit oxygen.

[0112]As a result, it is presumed that when the optical element is
irradiated with a blue laser, deterioration of the substrate material is
accelerated by a chemical reaction caused by the medium of oxygen and
that the amount of change in light transmittance increases. This
presumption conforms to the fact that a decrease in light transmittance
is relatively small when the uncoated optical element of the third
comparative example is placed in a nitrogen atmosphere.

[0113]More specifically, when a multi-layered film having a small oxygen
permeability coefficient is formed, the amount of change in light
transmittance of the optical element can be suppressed.

[0114]FIG. 24 illustrates a relationship between an oxygen permeability
coefficient and an amount of change in light transmittance. Referring to
FIG. 24, the amount of change in light transmittance is remarkably
lowered when the oxygen permeability coefficient of the optical element
is equal to or lower than 30 cm3mm/(m224 hratm).

[0115]FIG. 7 is a graph showing a change in chemiluminescence quantity
after a blue laser is irradiated on the optical element obtained by
forming the conventional film on the conventional substrate and the
optical element obtained by forming improved film 1 on the conventional
substrate and then stopped. FIG. 8 is a graph showing a part of FIG. 7 in
which a time axis is enlarged. In FIGS. 7 and 8, the chemiluminescence
quantity indicates a relative magnitude. For 20 seconds after the stop of
irradiation, the chemiluminescence quantity of the optical element
obtained by forming the conventional film on the conventional substrate
is larger than the chemiluminescence quantity of the optical element
obtained by forming improved film 1 on the conventional substrate. It is
said that chemiluminescence is caused by a reaction by the medium of
oxygen. It is considered that the optical element obtained by forming
improved film 1 on the conventional substrate suppresses a chemical
reaction of a substrate material caused by the medium of oxygen because
the optical element has an oxygen permeability coefficient smaller than
that of the optical element obtained by forming the conventional film on
the conventional substrate and does not easily transmit oxygen.
Accordingly, the optical element has the small amount of change in light
transmittance when having the small oxygen permeability coefficient.

[0116]FIG. 9 is a graph showing a quantity of fluorescence of the
conventional substrate and a substrate molded in a nitrogen atmosphere at
a wavelength of about 320 nm in excitation at a wavelength of 280 nm. The
substrate molded in a nitrogen atmosphere is a substrate which is molded
in a nitrogen atmosphere such that a thermoplastic transparent resin
cycloolefin polymer is dried in a nitrogen atmosphere and transported in
the nitrogen atmosphere. In FIG. 9, the quantity of fluorescence is an
arbitrary unit and indicates a relative magnitude. Since the fluorescence
is caused by the medium of oxygen, the reason why the quantity of
fluorescence of the substrate molded in a nitrogen atmosphere is small is
considered to be that the substrate molded in a nitrogen atmosphere has
an amount of absorbed oxygen smaller than that of the conventional
substrate.

[0117]FIG. 10 is a graph showing amounts of change in light transmittance
of the conventional substrate and the substrate molded in a nitrogen
atmosphere. The amount of change in light transmittance of the substrate
molded in a nitrogen atmosphere is considerably smaller than the amount
of change in light transmittance of the conventional substrate. In this
manner, the substrate molded in a nitrogen atmosphere is not easily
damaged by irradiation of a blue laser in comparison with the
conventional substrate.

[0118]It is considered that damage caused by irradiation of a blue laser
proceeds by a chemical reaction caused by the medium of oxygen.
Therefore, since the substrate molded in a nitrogen atmosphere has an
amount of absorbed oxygen smaller than that of the conventional
substrate, it is considered that the substrate molded in a nitrogen
atmosphere is not easily damaged by irradiation of a blue laser.

[0119]FIG. 11 is a graph showing amount s of change in light transmittance
of the optical element obtained by forming improved film 1 on the
conventional substrate and the optical element obtained by forming
improved film 1 on the substrate molded in a nitrogen atmosphere. Since
the substrate molded in a nitrogen atmosphere having a small amount of
absorbed oxygen and improved film 1 which does not easily transmit oxygen
are combined, the amount of change in light transmittance can be
suppressed to a very low level.

[0120]In the improved films 1 and 2, silicon dioxide is used as the
low-refractive-index material, and a tantalum oxide based material is
used as the high-refractive-index material. In addition to the silicon
dioxide, examples of the low-refractive-index material include metal
oxides such as aluminum oxide, metal fluorides corresponding to the metal
oxides such as magnesium fluoride, yttrium fluoride, ytterbium fluoride,
aluminum fluoride, calcium fluoride, and cerium fluoride, and a mixture
thereof. Examples of the high-refractive-index material, as alternate
materials of the tantalum oxide, include metal oxides such as titanium
oxide, aluminum oxide, yttrium oxide, hafnium oxide, cerium oxide,
zirconium oxide, niobium oxide, ITO, ytterbium oxide, magnesium oxide,
lanthanum titanate, and aluminum lanthanate, metal fluorides
corresponding to the metal oxides, and a mixture thereof.

[0121]Another embodiment in which a multi-layered film is formed by the
following film forming method will be described. The multi-layered film
which formed by the film forming method is referred to as improved film
3.

[0122]In the film forming method of Table 6, SiO2 is used as the
low-refractive-index material, and Ti3O5 that is of a titanium
oxide based material is used as the high-refractive-index material. As
described later, the oxygen permeability coefficient of the optical
element including the improved film 3 formed by the film forming method
of Table 6 is lower than that of the optical element including the
improved film 2 that is formed by the film forming method of Table 4 with
SiO2 as the low-refractive-index material and with the tantalum
oxide based material as the high-refractive-index material.

[0123]Still another embodiment in which a multi-layered film is formed by
the following film forming method will be described. The multi-layered
film which deposited by the film forming method is referred to as
improved film 4.

[0124]In the film forming method of Table 7, SiO2 is used as the
low-refractive-index material, and HfO2 is used as the
high-refractive-index material. As described later, the oxygen
permeability coefficient of the optical element including the improved
film 4 formed by the film forming method of Table 7 is lower than that of
the optical element including the improved film 2 or the improved film 3.

[0125]Still another embodiment in which a multi-layered film is formed by
the following film forming method will be described. The multi-layered
film which is formed by the film forming method is referred to as
improved film 5.

[0126]In the film forming method of Table 8, a mixture of SiO2 and
Al2O3 is used as the low-refractive-index material, and
LaxAlyO.sub.z (x, y, and z are a numerical value of 1 to 10) is
used as the high-refractive-index material. As described later, the
oxygen permeability coefficient of the optical element including the
improved film 5 formed by the film forming method of Table 8 is lower
than that of the optical element including the improved film 2, the
improved film 3, or the improved film 4.

[0127]In the film forming method of Table 8, the point that should be
noted is that the oxygen is not used as the ambient gas in forming the
low-refractive-index material and the high-refractive-index material. The
fact that the oxygen is not used as the ambient gas in the film forming
process is advantageous to lower the oxygen permeability coefficient of
the optical element.

[0128]For example, although TiO2 that is of the high-refractive-index
material has a stable ratio of bonding with the oxygen, Ti and O2
are easily decomposed by external energy. Therefore, the bonding oxygen
is deficient to become metal rich unless the oxygen is supplied, and
absorption is increased to form a film that is hardly used in optical
applications. Because the high-refractive-index material that can be used
without oxygen has a small oxygen decomposition ability, the
high-refractive-index material does not become metal rich even if the
oxygen is not supplied.

[0129]Examples of the high-refractive-index material with which the use of
the oxygen is not required as the ambient gas in the film forming process
include zinc sulfide (ZnS), cadmium sulfide (CdS), zirconium oxide
(ZrO2), hafnium oxide (HfO2), a mixture of zirconium oxide
(ZrO2) and titanium oxide (TiO2), lanthanum aluminate
(LaAlO3), a mixture of zirconium oxide (ZrO2) and aluminum
oxide (Al2O3), metal fluoride, and a mixture thereof.

[0130]Examples of the low-refractive-index material with which the use of
the oxygen is not required as the ambient gas in the film forming process
include silicon dioxide (SiO2), metal fluoride, zeolite
(Na2AlF6), chiolite (Na5Al3F14), cryolite
(Na3AlF6), dysprosium fluoride (DyF3), thorium fluoride
(ThF4), a mixture of silicon dioxide (SiO2) and metal oxide and
a mixture thereof.

[0131]A deoxygenating treatment is previously performed to the raw
material, because the titanium oxide and the tantalum oxide tend to be
decomposed into the metal and the oxygen to make the film forming
pressure unstable when a raw material is melt before the film forming.
Therefore, it is necessary to introduce the oxygen during the film
forming. For the aluminum oxide and the silicon dioxide, the
deoxygenating treatment of the raw material is not required because a
small amount of degassing is generated during the melting. Therefore, it
is not necessary to introduce the oxygen during the film forming. For a
material whose main component does not include the oxygen, it is not
necessary to introduce the oxygen during the film forming.

[0132]FIG. 17 illustrates oxygen permeability coefficients of the optical
elements in which the improved films 3 to 5 are formed on a conventional
substrate. The optical element in which the improved film 3 is formed has
the maximum oxygen permeability coefficient, and the optical element in
which the improved film 5 is formed has the minimum oxygen permeability
coefficient. The oxygen permeability coefficients of all the optical
elements are lower than that of the optical element in which the improved
film 2 of FIG. 6 is formed.

[0133]FIG. 18 illustrates a change in light transmittance of an optical
element in which the improved film 2 is formed on the conventional
substrate. A horizontal axis indicates wavelength, and a vertical axis
indicates the light transmittance of the optical element. FIG. 18
illustrates data before the optical element is irradiated with an
ultraviolet ray having the wavelength of 365 nm, data after the optical
element is irradiated with the ultraviolet ray for 1000 hours, and data
after the optical element is irradiated with the ultraviolet ray for 2000
hours.

[0134]FIG. 19 illustrates a change in light transmittance of the optical
element in which the improved film 4 is formed on the conventional
substrate. The horizontal axis indicates wavelength, and the vertical
axis indicates the light transmittance of the optical element. FIG. 19
illustrates data before the optical element is irradiated with an
ultraviolet ray having the wavelength of 365 nm, data after the optical
element is irradiated with the ultraviolet ray for 1000 hours, and data
after the optical element is irradiated with the ultraviolet ray for 2000
hours. The light transmittance after UV irradiation for 1000 hours is
increased for lights at wavelengths of 330 nm to 410 nm.

[0135]FIG. 20 illustrates a change in light transmittance of an optical
element in which the improved film 5 is formed on the conventional
substrate. The horizontal axis indicates wavelength, and the vertical
axis indicates the light transmittance of the optical element. FIG. 20
illustrates data before the optical element is irradiated with an
ultraviolet ray having the wavelength of 365 nm, data after the optical
element is irradiated with the ultraviolet ray for 1000 hours, and data
after the optical element is irradiated with the ultraviolet ray for 2000
hours. The light transmittance after UV irradiation for 1000 hours and
the light transmittance after UV irradiation for 2000 hours are increased
for lights at wavelengths of 330 nm to 440 nm.

[0136]In the optical elements whose data are illustrated in FIGS. 17 to
20, the improved films 2 to 5 are formed on the conventional substrate.
The deterioration of the light transmittance of the optical element in
which each of the improved films 2 to 5 is formed on a substrate molded
in a nitrogen atmosphere is less than that of each of the light
transmittances illustrated in FIGS. 18 to 20.

[0139]An attenuation coefficient of the light having the wavelength of 400
nm of the above-described materials having the little absorption of the
light in the ultraviolet wavelength range is equal to or lower than
1×10-2.

[0140]The attenuation coefficient is k in the following equation:

I = I 0 - α z α = 4 π
k λ ##EQU00003##

where,

[0141]I: light intensity in material

[0142]I0: initial light intensity

[0143]z: penetration depth into material

[0144]α: absorption coefficient

[0145]λ: wavelength of light

[0146]The improved films 1 to 5 are formed by the ion plating method.
However, in other film forming methods, the manufacturing conditions can
be changed according to the method of FIG. 12 to produce the optical
element having a small oxygen permeability coefficient.

[0147]FIG. 21 illustrates a configuration of a sputtering apparatus in
which film forming is performed by sputtering. A low-refractive-index
material metal target 513, a high-refractive-index material metal target
517, an ion gun 523 that is of an oxidation source, and a microwave
electron cyclotron resonance device (microwave ECR) 527 are provided in a
film forming chamber 501. The microwave ECR 527 includes a matching box
525. For example, a voltage at the ion gun is set in the range of 0 kV to
3 kV, and the output of ECR is set in the range of 0 W to 1500 W.

[0148]A substrate 551 is disposed in a drum 503. When the drum 503 is
rotated, the substrate 551 passes through positions facing the
low-refractive-index material metal target 513, the high-refractive-index
material metal target 517, and the microwave ECR 527.

[0149]First the method for forming the low-refractive-index material film
will be described. A voltage is applied to the low-refractive-index
material metal target 513 from a cathode power supply 511, and the argon
gas is supplied from an argon gas source 519 to form a
low-refractive-index material metal film forming region 553. A
high-frequency voltage is supplied to the microwave ECR 527 from a
microwave power supply, an ion beam is irradiated from an ion gun 523,
and the oxygen gas is supplied from the oxygen gas source to form a metal
film oxidation region 557. The metal film is formed when the drum 503 is
rotated to cause the substrate 551 to pass through the
low-refractive-index material metal film forming region 553. Then the
metal film is oxidized when the substrate 551 passes through the metal
film oxidation region 557. The film forming is continued until the
desired film thickness is obtained.

[0150]After the low-refractive-index material film became the desired film
thickness, a voltage is applied to the high-refractive-index material
metal target 517 from a cathode power supply 515, and the argon gas is
supplied from the argon gas source 519 to form a high-refractive-index
material metal film forming region 555 which will cover the substrate
551. The high-frequency voltage is supplied to the microwave ECR 527 from
the microwave power supply, the ion beam is irradiated from the ion gun
523, and the oxygen gas is supplied from the oxygen gas source to form
the metal film oxidation region 557. The metal film is formed when the
drum 503 is rotated to cause the substrate 551 to pass through the
high-refractive-index material metal film forming region 555. Then the
metal film is oxidized when the substrate 551 passes through the metal
film oxidation region 557. The film forming is continued until the
desired film thickness is obtained.

[0151]Thus, the metal oxide film made of the low-refractive-index material
and the metal oxide film made of the high-refractive-index material are
sequentially formed.

[0152]FIG. 22 is a view for explaining a principle of the film forming
performed by the sputtering. In the low-refractive-index material metal
film forming region 553 where the argon plasma is produced, sputtered
metal particles 561 adhere to the substrate 551. In the metal film
oxidation region 557 where the oxygen plasma is produced, the metal film
is oxidized on the substrate 551 to form a metal oxide film 563.

[0153]An example of the method for forming the film by the sputtering is
shown below.

[0154]In Table 9, the output of the oxidation source 1 indicates an output
of the microwave ECR 527, and the output of the oxidation source 2
indicates an output of the ion gun power supply 521 of the ion gun 523.
The oxygen flow rate of the oxidation source 1 and the oxygen flow rate
of the oxidation source 2 indicate oxygen flow rates supplied to the
matching box 527 and the ion gun 523, respectively.

[0155]FIG. 23 illustrates a change in oxygen permeability coefficient of
the optical element when the output of ECR of the sputtering apparatus is
changed during a period that the multi-layered film is formed on the
substrate made of thermoplastic transparent cycloolefin polymer.

[0156]FIG. 25 illustrates a configuration of an ion beam assist deposition
apparatus in which the film forming is performed by an ion beam assist
deposition method. In a vacuum chamber 601 of the ion beam assist
deposition apparatus, a base holder 607 that holds a base 608, an ion gun
603 that supplies the gas to the base 608 while the gas is ionized, a
neutralizer 604 that emits electrons in order to neutralize charges, and
a resistive heating board 606 to evaporate a evaporation material 605
that gives the thin film to the base 608 are provided. For example, an
anode voltage of the ion gun 603 is set at 50V.

[0159]In Table 10, Ti3O5 is used as the titanium oxide based
material by way of example. In Table 10, the letter EB designates the
electron beam heating.

[0160]During the film forming, the ambient gas such as the oxygen and the
argon is introduced into the vacuum chamber 601 through a valve (not
shown). The gas in the vacuum chamber 601 is evacuated through an exhaust
port 602.

[0161]A principle of the ion beam assist deposition method will be
described below. The ion gun 603 ionizes the introduced ambient gas, and
the ionized gas is supplied to the base 608. The dense film is deposited,
because the ionized gas supplied to the substrate gives energy when the
evaporated material is deposited on the base 608.

[0162]In the ion beam assist deposition method, as with the ion plating
method and the sputtering method, the oxygen permeability coefficient can
be lowered by changing one of the electric power (output of ion gun 603)
supplied to the ion gun 603, the ambient gas pressure in forming the
layer made of the low-refractive-index material, and the ambient gas
pressure in forming the layer made of the high-refractive-index material.

[0163]The invention is based on first knowledge that the conditions for
producing the optical element including the multi-layered film have the
large influence on the oxygen permeability coefficient of the optical
element and second knowledge that the significant correlation exists
between the oxygen permeability coefficient of the optical element
including the multi-layered film and the amount of change in light
transmittance for the light in the wavelength range of 300 nm to 450 nm
(that is, when the oxygen permeability coefficient is equal to or lower
than a predetermined value, the amount of change in light transmittance
of the optical element including the multi-layered film is suppressed to
a small value for the light in the wavelength range of 300 nm to 450 nm).

[0164]According to the invention, the conditions for producing the optical
element having the small amount of change in light transmittance can be
determined by measuring the oxygen permeability coefficient of the
optical element and determining the producing conditions such that the
oxygen permeability coefficient becomes equal to or lower than the
predetermined value.

[0165]As is clear from the first knowledge and the second knowledge, the
optical element producing conditions have the large influence on the
amount of change in light transmittance of the optical element.
Therefore, the conditions for producing the optical element having the
small amount of change in light transmittance for the light in the
wavelength range of 300 nm to 450 nm can also be determined by measuring
the amount of change in light transmittance of the optical element.

[0166]The optical element having the small amount of change in light
transmittance for the light in the wavelength range of 300 nm to 450 nm
can be produced by the producing method according to the invention.

[0167]The optical element of the invention acts as an antireflection
optical element, an optical filter, a beam splitter, and the like.